Inkjet recording apparatuses used as image recording apparatuses (image forming apparatuses), such as printers, facsimile apparatuses, copying apparatuses, and plotters, are equipped with an inkjet head as a droplet discharging head that includes: a nozzle for discharging ink drops; an ink channel (also referred to as a discharge compartment, pressure compartment, pressurizing compartment, liquid compartment, and so on) communicating with the nozzle; and pressure creating means for pressurizing ink in the ink channel. Droplet discharging heads also include, for example, a droplet discharging head that discharges a liquid resist in the form of droplets, and a droplet discharging head that discharges a sample of DNA in the form of droplets. In the following, however, a description will be given with focus on the inkjet head.
Inkjet heads such as the so-called piezo inkjet, the so-called thermal inkjet head, and an electrostatic inkjet head are known. The piezo inkjet deforms a vibrating plate that forms a wall surface of an ink channel by using a piezoelectric element as the pressure creating means for pressurizing ink in the ink channel, and varies the volume of the ink channel so as to discharge ink drops (refer to Japanese Laid-Open Patent Application No. 2-51734). The thermal inkjet head discharges ink drops with pressure that is created by generating bubbles through heating ink in the ink channel by using a heat resistive element (refer to Japanese Laid-Open Patent Application No. 61-59911). In the electrostatic inkjet head, the vibrating plate forming the wall surface of the ink channel and an electrode are arranged in a mutually opposing manner, and the vibrating plate is deformed by an electrostatic energy generated between the vibrating plate and the electrode, thereby varying the volume of the ink channel so as to discharge ink drops (refer to Japanese Laid-Open Patent Application No. 6-71882).
Some of such inkjet heads are driven by a push discharging method whereby ink drops are discharged by pushing the vibrating plate toward the pressurizing compartment so as to decrease the volume of the pressurizing compartment. In addition, some inkjet heads are driven by a pull discharging method whereby ink drops are discharged by deforming the vibrating plate with a force directed toward the outside of an ink compartment so as to increase the volume of the ink compartment and then bringing the vibrating plate to the original state so that the ink compartment is returned to its original volume.
Additionally, regarding the inkjet heads, the viscosity of ink is varied in accordance with temperature changes in different environments, which leads to speeding up or reducing of the speed (ink drop discharging speed) Vj of ink drops. Thus, the impact positions of ink drops on a recording paper may be shifted, and the volume (ink drop discharging volume) Mj of an ink drop may be increased or decreased. Consequently, the intensity of an image may be changed or image quality may be changed. Further, since the ink drop discharging speed Vj is increased and decreased, in some cases, injection bending occurs, and injection down accompanying the spray bending occurs.
Therefore, as described in Japanese Laid-Open Patent Application No. 11-268266, for example, as for a driving method of the piezo type head of the pull discharging type, taking environmental temperature changes into consideration, as shown in FIG. 1, a method is known in which a first signal P1 expands a pressure creation compartment, a second signal P2 maintains an expanded state of the pressure creation compartment, and a third signal P3 discharges ink drops by contracting the pressure creation compartment in the expanded state. Based on a temperature detection result of temperature detecting means, when the temperature is high, the difference between a first potential difference ΔV1 (that is, the potential difference between the first signal P1 and the second signal P2) and a second potential difference ΔV2 (that is, the potential difference between the third signal P3 and the second signal P2) is widened (increased). When the temperature is low, the difference between the first potential difference ΔV1 and the second potential difference ΔV2 is narrowed (decreased).
In other words, when the temperature is high, the potential of the first signal P1 and the potential of the third signal P3 are decreased as indicated by the broken lines in FIG. 1. On this occasion, by making the decreasing amount of the third signal P3 greater than that of the first signal P1, the difference between the first potential difference ΔV1 and the second potential difference ΔV2 is widened. On the other hand, when the temperature is low, the potential of the first signal P1 and the potential of the third signal P3 are increased as indicated by the two-dot chain line and the chain line in FIG. 1, respectively. At this point, by making the increasing amount of the third signal P3 greater than that of the first signal P1, the difference between the first potential difference ΔV1 and the second potential difference ΔV2 is narrowed.
However, in the conventional inkjet head driving method described above, when the temperature is high, the difference between the first potential difference ΔV1 and the second potential difference ΔV2 is increased, and when the temperature is low, the difference between the first potential difference ΔV1 and the second potential difference ΔV2 is decreased. Thus, when the temperature is low, the pressure creation compartment is contracted in a state where the meniscus is less pulled back than it is in ordinary temperature. Even if meniscus is pulled back, the pressure creation compartment is excessively contracted. Accordingly, the discharging volume Mj of an ink drop is increased.
That is, since the ink viscosity is varied in accordance with temperature, the ink drop discharging speed Vj is increased at high temperatures, while the ink drop discharging speed Vj is decreased at low temperatures. As indicated by the continuous lines in FIG. 2, however, the ink drop discharging volume Mj is increased both at high temperatures and low temperatures.
Here, if the difference between the first potential difference ΔV1 and the second potential difference ΔV2 is decreased, when the temperature is low, the pressure creation compartment is contracted in a state where the meniscus of the nozzle is less pulled back than it is at ordinary temperature. Even if the meniscus is pulled back, the pressure creation compartment is excessively contracted. Hence, the ink drop discharging volume Mj is increased as indicated by the two-dot chain line in FIG. 2.
As described above, in the conventional inkjet head driving method, there is a problem in that the ink drop discharging speed Vj and the ink drop discharging volume Mj are varied in accordance with temperature changes, resulting in degradation of image quality.